There are various processes that utilize radiation, for example, sterilization of materials; radiation therapy of biological subjects, including blood, people, and animals; food and crop irradiation for keeping properties or removal of pests, for example, pasteurization; alteration of material properties, for example, polymerization or cross-linking; quality checking, such as for cables, gel used in electrode surgery, and curing of inks in food labeling; and security scanning. These processes have a need to verify the administration of radiation, and/or the dosage of radiation administered.
There are a large number of different methods available to determine a dosage of radiation received by a subject or emitted from a material or machine, which methods are referred to as dosimetry. For example, methods of determining radiation doses can include, but are not limited to, ion dosimetry (ionization in air), calorimetry (for example, the determination of radiation-induced heat in carbon or metals), thermoluminescence dosimetry (luminescence in solids), and amino acid dosimetry.
The formation of radicals in solid organic substances on irradiation has been observed and the concentration of these radicals is proportional to the absorbed dose of radiation over a wide range of radiation doses. The radical concentrations can be determined easily by means of electron spin resonance (EPR) spectroscopy. Amino acids, for example, alanine, have been widely used for this purpose due to availability and the relative simplicity of incorporating amino acids into practical dosimeters.
Amino acid dosimetry is an accepted method to determine the radiation dose of different irradiation processes. On irradiating with ionizing radiation, radicals are produced in amino acids like alanine, for example, L-alanine or L-methylalanine, and the radicals are stable for long periods of time. The stability of the radicals is mainly due to the inhibition of radical-radical recombinations in the crystalline structure of the amino acid material, which prevents the migration of large molecule fragments. Non-destructive evaluation of the radical concentration in the amino acid can be done using EPR spectroscopy.
The use of EPR to determine an amount of radiation dose received by an amino acid requires a sensitive, robust and reliable instrument that can be serviced by a laboratory worker. A useful instrument provides such features as automated procedures for calibration and measurement of radiation dosages. Careful adjustment of an EPR spectrometer and the selection of suitable dosimeters allow the determination of dose rates in a range from 2 Gy to 200 kGy with a total uncertainty of 3.5% (confidence level of 95%).
Amino acid dosimeters are small, stable, and easy to handle. They are characterized by their large measuring range and a low sensitivity to temperature and humidity. This allows for their application in all types of radiation therapy, including the irradiation of blood, as well as in industrial facilities for irradiation of food and other goods. An advantage of the use of alanine dosimeters over inorganic dosimeter systems when measuring dosages applied to such organic materials is that the radiation absorption in the alanine-based dosimeters is closer to the radiation absorption in the organic materials being irradiated. This allows for improved dose measurement in such circumstances.
An amino acid dosimeter system can be used for reference and routine dosimetry due to its high quality and low costs. An example of an amino acid dosimeter is that described in U.S. Pat. No. 3,673,107, wherein the dosimeter includes amine salts or organic acids.
Alanine dosimeters are well known in the art. For example, in the reference: T. Kojima et al., “Alanine Dosimeters Using Polymers As Binders,” Applied Radiation & Isotopes, vol. 37, No. 6 (1986), Pergamon Journals Ltd., pp. 517–520, there are numerous references to alanine dosimeters made in pellet, rod, and film formats. Dosimeters have been made both by industrial laboratories and at academic institutions. These dosimeters can be in the form of molded pellets or rods. The alanine is generally blended with a synthetic or natural rubber, compounded, and molded under pressure to form a variety of shapes, as described, for example, in U.S. Pat. No. 4,668,714, JP 203276B, JP 01-025085A, and JP 61-578788. There are also references in the literature to extruded alanine films, as in JP 01-102388A. These extruded products, while working well, have several deficiencies. Their manufacture often requires the use of high pressures and temperatures during the molding process, requiring molding equipment that limits the sizes and shapes of available products. Molded dosimeters are also limited in that only moldable polymeric binders may be used. The use of molded dosimeters is also somewhat restrictive, as the size of the dosimeters tends to be very small, leading to difficulties in handling, and possibly loss during irradiation or subsequent handling of associated irradiated materials.
A potential solution to these difficulties would be an amino acid dosimeter formed of a length and width that allow easy handling, for example, coated onto a flexible support wherein the support serves to hold the amino acid and provide the user with a material that is easy to handle. Such a coated dosimeter has been described in DE 19637471A, wherein the alanine is coated from one of two specific binders, a polyoctenamer or polystyrene. Both of these binders are brittle materials and make the coating of thick alanine layers with good mechanical properties very difficult, especially when the thickness of the dosimeter layer is >100 microns. The ability to bend and shape the amino acid dosimeter coated on a support can be very important in some applications, and the lack of flexibility is a significant limitation of the coated dosimeters described in the art.
The response of an alanine dosimeter to ionizing radiation is proportional to the amount of alanine coated on the dosimeter. While within a given manufacturing batch the coated coverage may be very uniform, batch-to-batch variation makes it very important that dosimeters from a given batch be identifiable so calibration standards can be developed and used. Identification of one dosimeter from another can also be important, in case the dosimeter is separated from the goods with which it is meant to be associated. Such identification information for film dosimeters has been placed on film holders, as taught in U.S. Pat. No. 6,232,610.
Radiation processing and dose traceability are an integral part of the quality assurance of many fields, for example, medical products, including blood processing; food products, including tobacco, milk, and grains where the radiation is used for sterilization or pest control; and adhesives and inks, where the radiation is used for cross-linking or drying processes. Accurate quantitative dosimeters are important in optimizing radiation processing as well as in providing quality assurance for an item or product that has been irradiated. There is a further need to link the dosimeter to the irradiated product throughout manufacture, shipping, and possibly even usage by the consumer.
It would be useful to have a reliable method of measuring an absorbed dose of ionizing radiation, such as a dosimeter, wherein the dosimeter bears an identification mark that identifies the source and origin of the dosimeter, uniquely identifies the dosimeter from all other dosimeters, or both. It would further be useful to have a dosimeter that is flexible, easily handled, and reliably associated with a particular irradiated good.